Roland is still lagging in the sample quality department. They need to make drum modules that partner with one of the companies that specializes in samples:
Roland TD-12 sound vs Superior Drummer 2.0 - YouTube
I know that the TD20 is a better model, but Roland makes their samples in a similar way. Notice how the decay of the TD12 sounds are not quite right, and how exaggerated the rim-shot sound is. With SD2, the ROOM mics are real mics in the room that the drums were sampled. Also the cymbals on SD2 will have a more natural feel:
Superior Drummer 2.0 - NAMM 08 Demo PART1 - YouTube The making of the Superior Drummer 2.0 Sound Libraries - YouTube
The Superior Drummer series will give you the closest thing to walking into a good studio, having a good engineer, and recording real drums.
I'd also recommend Steven Slate Drums. They are not quite as "realistic" in a sense, but in exchange, they are basically mix ready (ie they have already been EQ'd etc). Lot's of pro guys use his samples layered on top of real drums when mixing. They are really something special....
If you prefer the SD type sound, but you are on a budget, then EZdrummer is decent too....but not as good...